1. Field of the Invention:
The present invention relates to a power conversion apparatus for use in an AC power source apparatus such as an uninterruptive power supply apparatus (hereinafter abbreviated to a "UPS") or a fuel cell generating system.
2. Description of the Related Art:
FIG. 8 is a block diagram which illustrates a power conversion apparatus disclosed in, for example, Japanese Patent Application No. 1-211737. Referring to FIG. 8, reference numeral 1 represents a DC power source, 2 represents an inverter circuit and 3 represents a transformer the input terminal of which is connected to the inverter circuit 2. Reference numeral 4 represents a cyclo converter circuit connected to the output terminal of the transformer 3. Reference numeral 5 represents a filter circuit connected to the output terminal of the cyclo converter circuit 4. Reference numeral 6 represents a load circuit, 10 represents a carrier signal generating circuit, 11 represents an inverter switching circuit, 12 represents a reference voltage signal generating circuit and 13b represents a switching signal generating circuit.
FIG. 9 illustrates the detailed structures of the inverter circuit 2, the transformer 3, the cyclo converter circuit 4 and the filter circuit 5. The inverter circuit 2 comprises switching devices S.sub.1 to S.sub.4 composed of transistors, MOSFETs or the like and diodes D.sub.1 to D.sub.4 which are connected respectively, in an anti-parallel manner, connected to the corresponding switching devices S.sub.1 to S.sub.4. The transformer 3 has a primary coil which is connected to the inverter circuit 2 and as well has a secondary coil which is connected to the cyclo converter circuit 4. The cyclo converter circuit 4 comprises switching devices S.sub.5 to S.sub.8 and S.sub.5A to S.sub.8A composed of transistors, MOSFETs or the like and diodes D.sub.5 to D.sub.8 and D.sub.5A to D.sub.8A which are connected respectively, in an anti-parallel manner, to the corresponding switching devices S.sub.5 to S.sub.8 and S.sub.5A to S.sub.8A. The two semiconductor switching devices S.sub.n and S.sub.nA (n=5 to 8) and two diodes D.sub.n and D.sub.nA (n=5 to 8) connected to the switching device in an anti-parallel manner constitute a bidirectional switch Qn which is capable of controlling the power supply direction.
As shown in FIG. 10, the inverter switching circuit 11 comprises a 1/2 divider 100 which transmits an output signal, the polarity of which is inverted in synchronization with the last transition of an input signal, and a NOT circuit 101 connected to the 1/2 divider 100. The inverter switching circuit 11 transmits switching signals T.sub.1 to T.sub.4 to the inverter circuit 2, the signals T.sub.1 l to T.sub.4 being signals for switching on/off the switching devices S.sub.1 to S.sub.4 of the inverter circuit 2.
FIG. 11 illustrates the detailed structure of the switching signal generating circuit 13b which comprises an absolute value circuit 102, a comparator 103, NOT circuits 105, 106, 108 and 110, 1/2 dividing circuits 104 and 107, a polarity discriminating circuit 109, AND circuits 111 to 118 and OR circuits 119 to 122. The switching signal generating circuit 13b transmits signals T.sub.5 to T.sub.8 which are signals for switching on/off switches Q.sub.5 to Q.sub.8 of the cyclo converter circuit 4.
Then, the operation of the above-described conventional apparatus will now be described with reference to a timing chart shown in FIG. 12. First, a sawtooth shape carrier signal Vp facing an upper right direction is transmitted from the carrier signal generating circuit 10. Then, switching signals T.sub.1 to T.sub.4, the duty ratio of each of which is 50%, are transmitted from the inverter switching circuit 11 shown in FIG. 10. That is, when the carrier signal Vp is supplied, signal Tx, which synchronizes with the signal Vp and which is halved, is transmitted from the 1/2 divider 100. Furthermore, the NOT circuit 101 transmits signal Ty which is a signal obtainable by inverting the sign of the signal Tx. As a result, the signal Tx is, as the switching signals T.sub.1 and T.sub.4, transmitted to the inverter circuit 2. Furthermore, the signal Ty is, as the switching signals T.sub.2 and T.sub.3, transmitted to the same. When the level of each of the switching signals T.sub.1 to T.sub.4 is high, the corresponding switching devices S.sub.1 to S.sub.4 of the inverter circuit 2 are switched on, while the same are switched off when the above-described level is low. Furthermore, the relationship between the switching on/off operations of the switching devices S.sub.1 to S.sub.4 and the secondary voltage V.sub.2 of the transformer 3 shown in FIG. 9 can be expressed as follows: EQU When the switches S.sub.1 and S.sub.4 are switched on: V.sub.2 =Vdc EQU When the switches S.sub.2 and S.sub.3 are switched on: V.sub.2 =-Vdc(A)
where symbol Vdc denotes the output voltage from the DC power source 1.
Therefore, the secondary voltage V.sub.2 becomes a rectangular wave voltage the duty ratio of which is 50% as shown in FIG. 12.
On the other hand, the reference voltage signal generating circuit 12 transmits reference voltage signal Vcc.sup.* serving as a reference of the voltage to be transmitted from the cyclo converter 4, the reference voltage signal Vcc.sup.* being, together with the carrier signal Vp, supplied to the switching signal generating circuit 13b. The switching signal generating circuit 13b receives the above-described signals so as to transmit the switching signals T.sub.5 to T.sub.8 the pulse width of each of which has been modulated as follows. Referring to FIG. 11, the reference voltage signal Vcc.sup.* is converted into absolute signal .vertline.Vcc.sup.* .vertline. by the absolute value circuit 102. The absolute signal .vertline.Vcc.sup.* .vertline. is, together with the carrier signal Vp, supplied to the comparator 103. The comparator 103 transmits signal Tp shown in FIG. 12, the signal Tp being then supplied to the 1/2 divider 104 in which the signal Tp is converted into signal Ta. On the other hand, when the signal Tp is supplied to the 1/2 divider 107 after the sign of it has been inverted by the NOT circuit 106, signal Tb formed into the same wave shape as that of the signal Tx is transmitted. Furthermore, when the signal Ta is supplied to the NOT circuit 105, signal Tc is transmitted, while when the signal Tb is supplied to the NOT circuit 108, signal Td formed into the same wave shape as that of the signal Ty is transmitted.
Then, the relationship between the signals Ta to Td and output voltage Vcc from the cyclo converter circuit 4 will now be described. In a case where there is a desire to make the polarity of the output voltage Vcc positive, the switching signals T.sub.5 to T.sub.8 are determined in accordance with the following equations: EQU T5=Ta, T6=Tc, T7=Td, T8=Tb (B)
In response to the above-described switching signals T.sub.5 to T.sub.8, the switch Qn (n=5 to 8) which constitutes the bidirectional switch is switched on/off. As a result, the output voltage Vcc from the cyclo converter circuit 4 is controlled. The fact that the switch Qn is switched on/off means that the switching devices Sn and SnA are simultaneously switched on/off. The relationship between the switching on/off operation performed by the switch Qn (n=5 to 8) and the above-described output voltage Vcc is expressed by the following equations: ##EQU1##
Therefore, the following facts can be deduced from Equations (B) and (C): when the levels of each of the signals Ta and Tb is high, the relationship Vcc=V.sub.2 is held, when the levels of each of the signals Tc and Td is high, the relationship Vcc=-V.sub.2 is held. When the levels of each of the signals Ta and Td or the signals Tb and Tc are high, the relationship Vcc=0 is held. Therefore, the output voltage Vcc from the cyclo converter circuit 4 is, as shown in FIG. 12, becomes positive voltage the pulse width of which has been modulated. In a case where there is a desire to make the polarity of the output voltage Vcc to be negative, the switching signals T.sub.5 to T.sub.8 may be determined in accordance with the following equations: EQU T5=Tc, T6=Ta, T7=Tb, T8=Td (D)
Then, the description about the operation shown in FIG. 11 will now be continued. The polarity discriminating circuit 109 transmits polarity signal Vsgn denoting the polarity of the reference voltage signal Vcc.sup.*. The NOT circuit 110 transmits a signal which is a signal obtainable by inverting the sign of the polarity signal Vsgn. The above-described signals and the signals Ta to Td are, via the AND circuits 111 to 118, supplied to the OR circuits 119 to 122. When the polarity of the reference voltage signal Vcc.sup.* is positive, the signals Ta, Tc, Td and Tb are transmitted from the AND circuits 111, 114, 116 and 117, respectively. Therefore, the switching signals T.sub.5 to T.sub.8 in accordance with Equation (B) are transmitted to the switches Q.sub.5 to Q.sub.8 of the cyclo converter circuit 4. Similarly, when the polarity of the reference voltage Vcc.sup.* is negative, the switching signals T.sub.5 to T.sub.8 in accordance with Equation (D) are transmitted to the switches Q.sub.5 to Q.sub.8. As a result of the above-described operations, the voltage Vcc, the wave form of which is obtainable by modulating the pulse width of the AC reference voltage signal Vcc.sup.* transmitted from the reference voltage signal generating circuit 12, is transmitted from the cyclo converter circuit 4. Furthermore, by supplying the above-described output voltage to the filter circuit 5 composed of a reactor L.sub.F and a capacitor C.sub.F as shown in FIG. 9, sine wave voltage from which the high frequency component has been removed by the pulse width modulation is supplied to the load circuit 6.
Since the conventional power conversion apparatus has been constituted as described above, there arises a problem in that undesirable surge voltage can be generated by energy stored in the circuit inductance because the electric passage is opened at the time of switching on/off the switching device of the cyclo converter circuit 4. For example, at time t1 shown in FIG. 12, a status in which the polarity of the voltage V.sub.2 is positive, the switches Q.sub.5 and Q.sub.8 are switched on and the cyclo converter circuit 4 is transmitting the positive voltage Vcc and being shifted to a status in which the switch Q.sub.5 is switched off, the switches Q.sub.6 and Q.sub.8 are switched on and the cyclo converter circuit 4 transmits zero voltage.
The switching device takes a long period of time to be actually switched on/off. Therefore, the switch Q.sub.6 must be switched on after the switch Q.sub.5 has been switched off in order to prevent an operational mode in which the switches Q.sub.5 and Q.sub.6 are simultaneously switched on and thereby the secondary terminal of the transformer 3 encounters a short circuit. However, since the circuit is temporarily opened during the shift from the switch Q.sub.5 to the switch Q.sub.6, the electric currents passing through the portions corresponding to the inductances of the filter circuit 5 and the load circuit 6 are cut off. Therefore, surge voltage is generated, causing a problem to arise in that excessively large voltage is applied to the switching device of the cyclo converter circuit 4 or the load circuit 6.
In order to overcome the above-described problem, an arrangement has been employed in which the switch Qn (n=5 to 8) of the cyclo converter circuit 4 is switched in only one direction as follows in accordance with the polarity of the output current from the cyclo converter circuit so as to perform the pulse width modulation in a manner similar to the above-described structure. For example, when the polarity of the output current from the cyclo converter circuit 4 is positive, the switching signals T.sub.5, T.sub.6, T.sub.7 and T.sub.8 are respectively applied to the switching devices S.sub.5, S.sub.6A, S.sub.7A and S.sub.8 and the residual switching devices S.sub.5A, S.sub.6, S.sub.7 and S.sub.8A are respectively switched off. When the polarity of the electric current is negative, the switching signals T.sub.5, T.sub.6, T.sub.7 and T.sub.8 are respectively applied to the switching devices S.sub.5A, S.sub.6, S.sub.7 and S.sub.8A and as well as the residual switching devices S.sub.5, S.sub.6A, S.sub.7A and S.sub.8 are respectively switched off.
However, the output current from the cyclo converter circuit 4 contains a ripple component generated due to the pulse width modulation. Therefore, its polarity changes to positive or negative when the quantity of the electric current is insufficient. As a result, the above-described switch selection cannot be performed satisfactorily, causing a necessity to arise in that an open state is created in the cyclo converter. As a result, surge voltage will undesirably be generated.
Accordingly, a great capacity snubber circuit must be included in the conventional power conversion apparatus for the purpose of absorbing the surge voltage. Moreover, the voltage rating of the switching device must be enlarged. Therefore, there arises a problem in that the size of the apparatus cannot be reduced or an excessively large loss is generated.